Y-cords (or, more correctly, Wye cords), can be useful when used properly. When used incorrectly however, they can lead to increased distortion, a dramatic drop in signal level, even damage to your device.
I've always known Y cords are problematic, and tried to avoid them whenever possible. I never really took the time to learn why (pardon the pun), however. This week, our lead sound engineer, Erik, sent me a link from the Rane website with a great article on the dangers of using Y cords improperly. For those impatient types, here's the executive summary: Using a Y cord to split one signal into two is OK; using a Y cord to combine two signals into one is evil.
It all has to do with impedance. Outputs are low impedance, inputs are high impedance. If you try to combine two low impedance outputs together, they drive each other into clipping. In other words, it's bad. So don't do it. Ever.
Personally, I'm not a fan of using Y cords even to split outputs. I once came a cross a signal chain that had a Y cord right at the board, and Y cords at every device along the chain—and there were 7 of them. That means the signal was split 7 times. It didn't work so good.
Anyway, here's a link to the Rane website so you can read the full article. They have some great diagrams on how to build an adapter that properly combines two inputs into one. If you'd rather just download the article in pdf format, click here and save it to your library. Thanks to Rane for making this available!